1. Field of the Invention
The present invention relates to a variable capacitor, a variable capacitor apparatus that uses this, a high frequency circuit filter, and a high frequency circuit. The variable capacitor and the variable capacitor apparatus can be used in, for example, wireless communication apparatuses or RF measurement apparatuses, etc.
2. Description of the Related Art
In the development of wireless communication technology such as portable telephones, the need for variable capacitors used in high frequency circuits, etc. is growing. Conventionally, a varactor, which is a semiconductor device, has been used as such a variable capacitor, but the Q value thereof is small, which brings about various nonconformities.
A variable capacitor that uses a MEMS (microelectromechanical system) has been proposed by Darrin J. Young and Bernhard E. Boser, “A Micromachined Variable Capacitor for Monolithic Low-noise VCOs,” Solid-State and Actuator Workshop, Hilton Head, June 1996, pp 86-89 (hereinafter “Young et al.”). This variable capacitor comprises a fixed electrode and a movable electrode arranged so as to form a parallel flat plate. The movable part is supported by a support part so as to be able to move with respect to the fixed electrode so that the gap between the two electrodes changes and so that a spring force by which the gap between the two electrodes attempts to return to the position at which the prescribed gap (initial gap) results is generated. The two electrodes act as both a capacity electrode for forming the capacity of the variable capacitor and a drive electrode for adjusting the gap between the two by generating electrostatic force that opposes the spring force.
However, in the variable capacitor disclosed above, the ratio of the maximum capacity value with respect to the minimum capacity value that can be obtained by variation could not be made very large. The reason for this will be described below.
In the variable capacitor disclosed above, the variable electrode stops at a position at which the electrostatic force between the fixed electrode and the movable electrode and the spring force are in balance. The spring force is proportional to the amount that the gap between the two electrodes has changed from the initial gap. On the other hand, the electrostatic force is proportional to square of the voltage between the two electrodes and is inversely proportional to square of the gap between the two electrodes.
Therefore, when the voltage between the two electrodes is increased, from when the gap between the two electrodes is at the initial gap until it becomes ⅓ of that gap, the spring force and the electrostatic force stabilize and become balanced, and the movable electrode stabilizes and stops at an electrode gap corresponding to the applied voltage. On the other hand, when the voltage between the two electrodes is increased, and the gap between the two electrodes narrows beyond a gap that is ⅓ of the initial gap, it is not possible for the spring force and the electrostatic force to stabilize and become balanced, and even if the voltage between the two electrodes were not further increased, the electrostatic force would exceed the spring force at any position at which the gap between the two electrodes is narrower than a ⅓ gap. For this reason, when a voltage that is larger than a voltage that would result in the gap between the two electrodes becoming a gap that is ⅓ of the initial gap is applied, regardless of the magnitude of that voltage, a so-called pull-in phenomenon occurs, in which the movable electrode approaches the fixed electrode up to the limit.
For this reason, the range of the gap between the two electrodes that can be continuously adjusted by means of a voltage applied between the two electrodes (continuous adjustment range) is limited to a range from the initial gap to ⅓ of that gap. Therefore, in the variable capacitor disclosed above, only variation from the capacity value at the time of the initial gap up to a capacity value that was 1.5 times that capacity value was possible.
Therefore, Japanese Unexamined Patent Application Publication No. H9-199376 and U.S. Pat. No. 6,593,672 propose a variable capacitor apparatus that makes it possible to obtain the desired capacity variation range, such as by widening the capacity variation range, and that combines a plurality of fixed or variable capacitors and a switch that is specially provided in addition thereto.
The variable capacitor apparatus disclosed in Japanese Unexamined Patent Application Publication No. H9-199376 (in Japanese Unexamined Patent Application Publication No. H9-199376, called a “variable capacity condenser”) comprises first through third fixed capacitors comprising two opposing electrodes leaving a space and first and second MEMS switches. In addition, a series circuit of a first fixed capacitor (the capacity thereof is C1), a second fixed capacitor (the capacity thereof is C2) and a first MEMS switch and a series circuit of a third fixed capacitor (the capacity thereof is C3) and a second MEMS switch are mutually connected in parallel. According to this variable capacitor apparatus, it is possible to vary the obtained compound capacity to the respective capacity values of C1, C1+C2 and C1+C2+C3 by means of the ON-OFF statuses of the first and second MEMS switches.
The variable capacitor apparatus disclosed in U.S. Pat. No. 6,593,672 comprises plurally connecting, in parallel, series circuits of MEMS capacitors and MEMS switches as the variable capacitors. According to this variable capacitor apparatus as well, it is possible to vary the compound capacity obtained to various values by varying the ON-OFF statuses of the respective MEMS switches. The variable capacitor itself used in the variable capacitor apparatus of U.S. Pat. No. 6,593,672 is similar to the variable capacitor disclosed in Young et al.
However, in the conventional variable capacitor apparatus such as discussed above, a special switch was required in addition to the fixed or variable capacitor, so the occupied area increased, the Q value decreased due to contact resistance between the switch contact points or the Q value decreased due to the resistance of the wiring that connects the capacitor and the switch.
In addition, depending on the application, obtaining only a two-value capacity may be sufficient, but there are also cases in which it is necessary for the ratio of two capacity values to be large. However, in the variable capacitor disclosed in Young et al. and the variable capacitor itself used in the variable capacitor apparatus disclosed in U.S. Pat. No. 6,593,672, for reasons such as those discussed above, it was not possible to obtain a two-value capacity with a large mutual ratio. Therefore, for example, connecting in parallel a series circuit of a first fixed capacitor, which has an adequately small capacity value, a second capacitor, which has a relatively large capacity value, and a switch is conceivable. In this case, while it is possible to obtain an adequately small capacity value by turning the switch off, it is possible to obtain a relatively large capacity value by turning the switch on, and it is possible to obtain a two-value capacity with a large mutual ratio. However, in this case, in requiring two fixed capacitors, a special switch is required, so an increase in the occupied area and a decrease in the Q value are unfortunately brought about.